Although it's more manual, you could set up a mic to record into Audacity, cut the sections with no sound, and the length of the clip should give you your answe

Although it's more manual, you could set up a mic to record into Audacity, cut the sections with no sound, and the length of the clip should give you your answer. Or if the piano supports midi, maybe record using some kind of DAW (e.g. FL Studio), that way you won't be fooled by someone making noise into the mic to keep the timer going.